The gasoline- or diesel-powered motor vehicle with an internal combustion engine is the standard mode of conveyance for the majority of adults in civilized countries. Unfortunately, such motor vehicles with engines operating on gasoline or other hydrocarbon fuel have two significant disadvantages. First of all, the exhaust emissions from vehicles is a significant contributor to the air pollution problem in urban areas. Second, most countries do not have sufficient natural resources to produce hydrocarbon fuels (particularly gasoline) at or near market prices. Accordingly, these countries are dependent upon other countries for these resources.
For these and other reasons, momentum is building to develop motor vehicles with alternative sources of power. Among the leading candidates are electric vehicles which are powered by electrochemical batteries. In the United States, at both the state and federal levels, there are current laws and pending legislation relating to: the sale of new electric vehicles; tax credits for purchasers of electric vehicles; and requirements on the percentage of emission-free vehicles which must be sold relative to vehicles which do exhaust emissions. In 1990, the Federal Government authorized the establishment of the U.S. Advanced Battery Consortium (USABC). Under the aegis of the Department of Energy, USABC brings together Chrysler, Ford, General Motors, and the Electric Power Research Institute to sponsor research and development of batteries for electric vehicles. The USABC has identified a number of parameters, or goals, for an electric vehicle battery system.
A very basic requirement is that the battery system must be rechargeable. Aside from that, one of the most important parameters relates to the energy density of the battery system (as used herein, energy density is the total available energy per unit of mass). Since batteries inherently have much lower energy densities than other sources of energy such as fossil fuels, much of the research and development in the battery industry has related to maximizing the energy density by experimenting with new reagents involving lighter chemicals in the basic electrochemical process. Thus, batteries based on Lead-acid have been replaced with Nickel-Cadmium (NiCd) batteries, found in many consumer product applications. In space vehicles, Nickel-Hydrogen (NiH.sub.2) batteries have been used. Unfortunately, the gaseous nature of the electrochemical reaction in an NiH.sub.2 battery necessitates the use of a pressure vessel to contain the battery. Further, in the presence of oxygen, such as within the Earth's atmosphere, NiH.sub.2 batteries have numerous safety issues relating to the flammability of the hydrogen.
Much improved battery systems based on other reagents are currently being developed. For example, some consumer products are currently supplied with Nickel-Metal-Hydride (NiMH) batteries in a further attempt to maximize the energy density. In addition, Lithium ion (Li.sup.+) batteries are currently in development. Each of these types of batteries offer the advantages of increased energy density and non-toxic ingredients as compared with the older batteries using lead or cadmium.
No matter what reagents are employed for an electrochemical battery, there is a theoretical limitation with regard to energy density. That is, each molecule can give up only one electron and the voltage potential of that electron is limited by the nature of the ion created. Thus, even with the lowest atomic weight possible in a molecule, there can only be a single electron generated per molecule. This theoretically places an upper limit on the energy density of electrochemical batteries. The Lithium ion battery has the largest theoretical energy density of any of the previously-discussed reagents in use because of the low atomic weight of Lithium and high voltage potential of the Lithium ion.
Currently, one of the leading candidates for an electric vehicle battery system is for Nickel-Metal-Hydride (NiMH) batteries. A current goal for NiMH batteries is an energy density of approximately eighty Watt-hours per kilogram. By way of comparison, gasoline has an energy density on the order of magnitude of 3,000 Watt-hours per kilogram. In other words, one kilogram of gasoline can produce over thirty times as much energy as one kilogram of the projected NiMH battery.
Because of this relatively low energy density in batteries, the battery must have a very large mass. Thus, the battery system for an electric vehicle will be extremely large in volume and mass, perhaps occupying most of the engine and trunk compartments of a standard passenger automobile. As can be seen, the battery not only has a large volume and mass but is concentrated into one or two particular areas in the vehicle. Such a design can be dangerous in an automobile crash in which the concentrated, large battery may come through the trunk of the vehicle and crush the occupants and contents in the passenger compartment of the vehicle.
Typically, electrochemical batteries are placed in an outer shell or container which adds nothing to the operation or function of the battery or of the vehicle or device associated with the battery. The container is filled with stacked metal plate electrodes or a jelly-roll configuration of adjacent electrodes. As battery systems become very large for electric vehicles, the mass of this container becomes substantial. Even for relatively smaller power applications, such as space vehicles, consumer electronics, and power tools, the container itself can be viewed as wasted space and extra mass and cost. In certain space vehicles, elimination of the battery container could result in a significant mass reduction of ten to fifty percent of the battery mass.